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Fill Under Slab



Ensure soil is compacted

Even though it should have previously been done, it is worth checking the soil at the bottom of the footings is properly compacted.  If you have been careful not to over excavate and if you are lucky with the solid type then it may not be necessary to use a compactor on the soil.  It needs to be 95% or better relative compaction and it is possible to have it tested if you are unsure about how compacted it is.

Check Form-a-drain relative to batter board strings

It's always good to periodically check that the Form-a-drain is in the right position using the plumb bobs and at the right height.  It is particularly worth checking the outer Form-a-drain is 24" below grade (in my case that's also 24" below the batter board strings).  The top edge of the outer Form-a-drain is the top of slab height.


Getting aggregate

Get aggregate delivered

The best gravel to use depends on what's available in your area, but in my area it is "1.25 inch crushed rock without fines".  This has jagged edges so locks together when compacted, so forms a good stable foundation base.  Because the bits of crushed rock are a decent size it does allow water to drain through.  It can be used as a substitute for drain rock, but you should ideally use proper drain rock (with it's rounded stones) to surround pipes so there is no danger of the sharp edges puncturing the pipe.  In my area the drain rock is called "1.25 inch washed drain rock".

Crushed Rock 1-1/4 Minus Without Fines 1.25 inch crushed rock without fines

Typically you need to order aggregate by the truck load.  That means you get something like 16 - 18 tons at a time of your chosen aggregate.  It is assumed that you have both drain rock and crushed rock available on your site.  The trucks will have dumped them on your site in a place of your choosing in two big piles.  Note that aggregate trucks are very big so you will need to provide plenty of access for them to get to the place where you want it dumped.

Truck dumping aggregate 

Implement a shoot pipe

You need to get the aggregate from the pile where the truck dumped it to the place where it's needed.  That will likely involve lots of back breaking shoveling and carrying in buckets, but in some cases you may be able to make yourself a u shaped shoot made out of eg lumber with a lining of metal roof sheeting

Crushed rock shoot pipe 


Fill under-slab area

Fill the under-slab pipe ditches with drain rock

Fill the pipe ditches right up to the height of the under-slab excavation.  Make sure the pipes are well supported and well surrounded.  Use a manual tamper tool to get it fairly well compressed in there.

As long as proper smooth stone drain rock is used round the pipes, you can get away with using crushed rock above that for filling in the bulk of the ditch.

Put geo-fabric over the under-slab area

The geo-fabric is just a safety layer of protection.  Another thing to help dissuade termites.  Geo-fabric lets water through ok.


Drape it down the gully between the edge of the under-slab earth mound and the Form-a-drain.

Geo-fabric over slab area 

Fill behind the inner Form-a-drain with crushed rock

Completely fill with "1.25 inch crushed rock without fines" up the inch or so gap between the inner Form-a-drain and the center soil mound that is under the slab.  You will need to trickle it down there.  Do not over force it or over compact it as that may make the Form-a-drain bulge out.  Fill up to the under-slab excavation height.  The crushed rock will be retained by the top and bottom inner Form-a-drain and the polystyrene plus PVC filler strip between the two.

It is ok to use crushed rock as the slots in the Form-a-drain are thin and the forces on the Form-a-drain are small so it won't be punctured (and it wouldn't matter even if it was punctured).  Crushed rock is the best choice because it interlocks and is more structural than drain rock, so it will help resist lateral forces in an earthquake.


Put a total of 6" of crushed rock over slab area

Hire a compactor

You need to hire one that is 5HP or more.  Ideally you need to add the crushed rock in layers and compact between the layers but that is a bit inefficient on hiring a compactor, so you may decide to add all the crushed rock first before hiring the compactor.

Add first half (3") of crushed rock

This is the first half of the gravel (1.25" crushed rock without fines).  Ideally the crushed rock is added in two layers to help get it well compacted.

Compact first layer of crushed rock

You need to compact the gravel that will be under the concrete slab using the hired compactor.  Even though the inner Form-a-drain is in place and back filled with drain rock, you still need to go easy with the compactor near the footing trench edges because you don't want to move the Form-a-drain from its correct location.

Add second half (3") of crushed rock

This is the second layer of crushed rock to bring it up to a total of 6 inches.

Compact second 3" of crushed rock

Compact this layer.  If the overall thickness of crushed rock dips below 6" then add more crushed rock as needed.  The height needs to end up a uniform 1" below the top of the inner top Form-a-drain.

Return the hired compactor

The hired compactor is no longer needed for the building, but it may be of use for any extra crushed rock used on driveways or pathways in the yard.

Job site with outer EPS installed 


Fill behind lower outer Form-a-drain

Necessary to balance forces

This is crushed rock over and around the 4" perforated drain pipe that goes round the outside.  You should fill with enough crushed rock at this stage to come up to about 1 foot above the top of the lower Form-a-drain .


Note that more crushed rock will be added later once the EPS at the bottom of the footing is in place as with the extra EPS it can withstand more force from the crushed rock.  The reason some is added ahead of the EPS footing lining is that it is easier to add crushed rock without having to be careful not to damage the EPS and membrane linings.

Don't forget that the wooden jigs (if you used them) will be removed before the concrete is poured so you will be relying on the wiring to the stakes and the polystyrene that will be lining the footing trench.  You may want to wait until the polystyrene footing lining is in place and the jigs have been removed before adding the last bit of crushed rock round the outside.


If you have implemented a shoot to get the crushed rock to the under-slab area then you can shovel rock from the center to the outside.  For greater amounts of crushed rock you can re-position the chute to deliver rock to buckets in the bottom of the footing trench.


Treat ground under footings and slab for termites (Optional)

Done just before polyethylene sheet

You can buy termite treatment from your local home center in the gardening department.  Do it just before putting the polyethylene sheets on top so that evaporation is avoided.  Don't let rain wash it away.

Only lasts a year

Note that termite soil treatment only lasts for about a year, and the house is designed to last for 500 years, so it really is of fairly marginal value to treat the soil for termites.  If you are in a high termite area then you might consider once a year blocking up the under slab drainage pipe at the house exit using an inflatable bladder and pouring termite treatment down the cleanout pipes and radon vent pipes.

I decided not to do

My main defense against termites burrowing through the polystyrene will be the 6 mill polyethylene sheets.  Also there is no wood used in the house structure so there is little to attract termites.  I decided not to bother with the termite poison.  If I ever change my opinion then I will retrofit it using the drainage pipes under the building.